Photosensitization of printing masters

ABSTRACT

Printing masters are formed by a photosensitization and selective conversion process. A surface material, preferably a ZnO resin binder system is provided which is normally hydrophobic/oleophilic in nature but which can be converted to hydrophilic/oleophobic in nature by chemical reaction. The surface material also is selected such that the rate of its conversion can be altered by exposure to radiation energy. Selected areas only are exposed to the radiation (heat may also be applied), to form a more reactive surface area where exposure has taken place. The entire surface is then contacted with a conversion reagent for a sufficient time to convert the exposed areas to a far greater extent than the non-exposed areas. The selectively convertible master then is used to perform the printing function.

United States Patent Tandy [451 Feb. 20, 1973 [54] PHOTOSENSITIZATION OF PRINTING Primary Examiner-David Klein MASTERS Attorney-Russell L. Root and Ray S. Pyle [7 5] Inventor: William H. Tandy, Mentor, Oh1o 57] ABSTRACT [73] Asslgnee: ggigzi fg g fi Printing masters are formed by a photosensitization p e a and selective conversion process. A surface material, [22] Filed: April 7, 1971 preferably a ZnO resin binder system is provided which is normally hydrophobic/oleophilic in nature [21] Appl' 132222 but which can be converted to hydrophilic/oleophobic in nature by chemical reaction. The surface material [52] U.S. Cl ..96/33, 96/88, 96/90 PC, also is Selected Such that the rate of its conversion can 101/465 be altered by exposure to radiation energy. Selected 51 1m. (:1. ..c03r 7 02, G030 1/00, G036 1/52 areas ("11y P the radiatitm (beat my 58 Field of Search ..96/33, 88,90 PC; 101/465 be applied), m a reactive surface where exposure has taken place. The entire surface 1s [56] References Cited then contacted with a conversion reagent for a sufficient time to convert the exposed areas to a far UNITED STATES PATENTS greater extent than the non-exposed areas. The selectively convertible master then is used to perform the 3,615,559 10/1971 Kaspaulet al. ..96I88 priming function 3,573,040 3/1971 Back ..96/33 UX 3 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relates generally to the production of printing masters, and more particularly to the production of printing masters by photosensitization and selective conversion.

One common method of producing lithographic printing masters is by the electrostatic process. In this process, a master sheet coated with a photoconductive material is selectively charged to provide the desired latent image thereon. A toner is then applied to this surface, which toner will adhere to the charged surface where the latent image is formed. The adhering toner is then fixed at elevated temperatures which will set the toner on the surface, and the entire surface of the master is treated with a conversion solution. A conversion solution converts the non-imaged areas chemically, i.e., the background photoconductive material, to a water insoluble hydrophilic material, and the fused toner in the form of the image remains hydrophobic. When ink is applied, a hydrophilic background material is oleophobic and the fused toner is oleophilic, thus the master will receive the ink in the imaged areas and print.

There are several disadvantages to the above described prior art process. One disadvantage is, of course, the number of separate steps which must be performed in order to produce the desired printing master. Each of these separate steps requires a processing station and associated equipment and devices for producing masters according to this process. Also, each of these separate and distinct steps imposes certain physical and chemical limitations on the materials which are useful, thereby limiting the range of materials and processing steps which are available and which can be used in combination with each other.

There also have been many prior art proposals for photographic processes of producing printing masters. Basically, these processes utilize light sensitive photographic material, and photographs are made on the master which is then developed by conventional photographic developing techniques, after which the surface is converted to the proper oleophilic/hydrophobic, hydrophobic/oleophilic nature for printing. These photographic processes also entail several steps and are somewhat complex.

SUMMARY OF THE INVENTION According to the present invention a process for forming a printing master is provided which utilizes a surface material which is normally hydrophobic/oleophilic in nature but can be converted to hydrophilic/oleophobic in nature by a chemical reaction. The selected surface material is such that the rate of conversion can be varied by exposure to radiation energy of the proper wave length. Then, only selected areas are exposed to an energy source of the radiation energy. Thereafter, the surface is contacted with the chemical conversion reagent for a sufficient time to convert the more reactive areas, to a far greater extent than the less reactive areas. The selectively converted material is then utilized for the printing function.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention, in its preferred form, contemplates the provision of a process of forming a printing master wherein a base material such as a sheet of treated paper, or plastic, or other suitable material, is coated with a material which is normally hydrophobic/oleophilic in nature but can be converted to hydrophilic/oleophobic in nature by chemical reaction with a chemical reagent such as various soluble phosphates. This convertible material is also selected such that it has the characteristics that its reaction rate or reactivity with the chemical reagent is increased by exposure to a given radiation energy.

To sensitize this material, a transparency is provided which has the darkened areas thereof corresponding to the desired printing areas on the master. The treated master is then exposed to the source of radiation energy through the transparency, so that the radiation will pass through the transparent area, thus sensitizing that portion of the master, while blocking the remaining portions of the master from the radiation energy. The

thus sensitized master is then placed in a conversion solution, such as a phosphate, for a sufficient time to convert the exposed sensitized areas of the master to a hydrophilic/oleophobic nature but an insufficient time to convert the non-sensitized areas, thereby allowing these areas to remain oleophilic/hydrophobic. The master when thus exposed and converted can be placed on a printing apparatus and inked and printing performed in a conventional manner.

There are several suitable materials for forming the coating on the master. The preferred material, however, is a mixture of zinc oxide particles as a pigment in an acrylic binder resin such as the type sold under the trade name Acryloid B 72 by Rohm & Haas Corporation, the acrylic resin are mixed in a ratio of approximately 16 to 1, pigment to hinder ratio, and applied to the paper as a coating.

A transparency of the desired printing configuration is provided wherein the printing areas are dark and the background is transparent. The master is then exposed through the transparency to a light source in the ultraviolet region of approximately 3800 angstroms or less. This process will be referred to as photosensitizing the material to energy to change its reaction rate. Exposure times of at least approximately 2.5 to 30 seconds are required to get suitable photosensitization to effectively change the reactivity rate of the material, although in some instances longer times may be necessary. The time will be governed by such factors as the intensity and wave length of the light as well as by the specific pigments and binders used.

It has further been found that if the material is heated during exposure to the energy, a lesser amount of light energy is required to cause the photosensitization. This energy decreases up to about C. while heating above this temperature does not materially further reduce the amount of light energy required to photosensitize the material. Therefore, it is also preferred to heat the master up to the range of 1 15 to 1 30 C. during exposure. a

After the material has been selectively photosensitized, the master is then brought into contact with the conversion reagent which is a water solution of a soluble phosphate such as an ammonium or a sodium phosphate. The master is left in the conversion solution a sufficient time to convert the photosensitized areas from their normal hydrophobicloleophilic nature to a hydrophilic/oleophobic nature but an insufficient time to convert the shaded or unsensitized areas thereby allowing them to remain oleophilic/hydrophobic. The master is then ready for printing in a conventional manner. (It is possible that some conversion of the shaded areas does take place, but of insufficient degree to be detrimental.)

The reason why this particular combination of zinc oxide and binder when exposed to light has its conversion rate of reaction increased, is not completely understood. One possible explanation, is that the light energy acts upon the zinc to increase its energy level, thereby causing it to be more reactive in nature than the nonexposed zinc.

Another possible explanation is that the energy from the light source supplies the required energy to initiate a reaction between the zinc oxide and the components of the binder and/or the surrounding atmosphere to produce a reaction product which will convert more rapidly to a hydrophilic/oleophobic material than the unexposed zinc oxide binder system. it is possible that intermediate reaction products such as ZNO'OH, 0,, H and water are formed, and it is also possible that a zinc-organoperoxide is formed by a reaction of the zinc oxide to the binder. The zinc, and combination of zinc products, may react much more rapidly with the phosphate conversion solution to form the conversion product than the zinc oxide. This also would explain the photosensitizing phenomenon.

It has also been found that the preferred zinc oxide resin binder system requires light in the ultraviolet frequency range to have effective photosensitization within a satisfactory period of time. Longer wave lengths of light do not seem to be particularly effective in the sensitizing process. The reason for this is not understood, although the light of 3800 angstrom wave length corresponds to a photon energy of 3.28 ev., the band gap energy of ZnO. This would appear to be significant in terms of energy requirements for chemical reaction. Also, while applied heat increases the rate of sensitization by light in the ultraviolet range, heat itself does not induce a photosensitization by itself in the preferred system. Thus, light energy, while inducing a photosensitization, must be in the proper range, and if in the proper range, the application of heat will greatly decrease the time required; or expressed another way, will decrease the light energy required to form a proper photosensitization.

Also, it has been found that the photosensitized master can be stored for extended periods of time, i.e., one year or longer, and still retain its photosensitized characteristics so that if the master, after storage, is placed in contact with a conversion reagent, the photosensitized areas will preferentially convert in the same manner as immediately following photosensitization.

While at the present time the preferred material is zinc oxide pigment in an acrylic resin binder, it is to be understood that other binders and pigments can be used. For example, with zinc oxide pi ment, such binders as alkyds, epoxies, phenohcs, p0 yketones, and as well as coumarone-indene resins can be used. It has been found, however, that one characteristic of the resin in all types that have been tried is that it has an oxygen containing group in its structure. This would seem to indicate that there is a very strong possibility that some type of chemical reaction is taking place involving the oxygen groups to form a more reactive substance.

With respect to the pigment material, zinc oxide has proved to be the most effective material of those tried. However, other materials are also useful. For example, Ti0 can be used, although a conversion solution of oxalic acid is required rather than the phosphate utilized for converting zinc oxide. Other potentially useful pigments include PbO, CdS, MgO, and ZnS. In this respect, it is to be understood that the essential requirement of the material, i.e., the pigment and binder system, is that it be convertible from hydrophobic/oleophilic to hydrophilic/oleophobic and that this conversion rate be varied by exposure to some form of radiant energy.

It is readily apparent that the process of producing printing masters by the present invention is a substantial improvement over prior art methods. Basically, all that is required in the present method is that the coated surface of the master be exposed in that area thereofwhich is to be background nonprinting surface to provide photosensitized material, and thereafter the master brought into contact with a conversion solution after which the master is ready for printing. There are no complex complicated steps involved such as in the photographic processes or in the electrostatic processes of forming the master.

What is claimed is:

1. A method of preparing a printing master comprising the steps of providing a master blank having an oleophiliclhydrophobic surface coating formed of a zinc oxide pigment in a binder resin consisting essentially of an acrylic ester resin exposing selected portions of the surface coating of said master blank to ultra violet radiation of 3800 angstroms and less thereby sensitizing said selected portions contacting said surface coating with a phosphate converting solution for a time sufficient to convert the sensitized portions of said coating to an ink repelling oleophobic/hydrophilic character while the unsensitized portions of said surface coating remain relatively oleophilic/hydrophobic and ink receptive.

2. The method defined in claim 1 wherein said zinc oxide pigment and said acrylic ester resin are present in a ratio of about 16:1.

3.. The method defined in claim 2 wherein the irradiation of said master blank is conducted of an elevated temperature within the range of about C to about C for an exposure period of from about 2.5 to about 30 seconds. 

1. A method of preparing a printing master comprising the steps of providing a master blank having an oleophilic/hydrophobic surface coating formed of a zinc oxide pigment in a binder resin consisting essentially of an acrylic ester resin exposing selected portions of the surface coating of said master blank to ultra violet radiation of 3800 angstroms and less thereby sensitizing said selected portions contacting said surface coating with a phosphate converting solution for a time sufficient to convert the sensitized portions of said coating to an ink repelling oleophobic/hydrophilic character while the unsensitized portions of said surface coating remain relatively oleophilic/hydrophobic and ink receptive.
 2. The method defined in claim 1 wherein said zinc oxide pigment and said acrylic ester resin are present in a ratio of about 16:
 1. 